A vehicle may be equipped with a lean-burn gasoline or diesel engine, or an engine configured to consume an alternative fuel (alcohol, biodiesel, etc.) under lean conditions. An exhaust system in the vehicle may include a lean nitrogen-oxide trap (LNT), configured to sequester nitrogen oxides (NOx) from the exhaust stream during lean operation. The exhaust system may also include a selective catalytic reduction (SCR) device, configured to catalyze the reduction of NOx to dinitrogen via nitrogen-containing reducing agents (e.g. ammonia) entrained in the exhaust stream during lean operation.
The capacity of the LNT for continued sequestration of NOx may be intermittently restored via a regenerative enrichment phase, wherein the engine is temporarily induced into providing rich (i.e., reducing) exhaust. In some configurations, the LNT and the SCR device may be disposed so that ammonia released from the LNT during the enrichment phase flows to the SCR device to reduce NOx therein. In addition, some ammonia released from the LNT during the enrichment phase may be stored in the SCR catalyst and used during lean periods to reduce NOx that may slip past the LNT.
Further, the engine may be operatively coupled to a controller. The controller may be configured to initiate the regeneration phase by temporarily causing the engine to provide rich exhaust. In some examples, the controller may be further configured to stop the regeneration phase after a predetermined interval, by restoring the engine to lean operating conditions. In other examples, the controller may stop the regeneration phase pursuant to a sensor response.
For example, U.S. Patent Application Publication Number 2007/0033928 describes an LNT/SCR system for a vehicle, in which a sensor is disposed downstream of the LNT and upstream of the SCR device. The reference further describes, in one specific example, that an enrichment phase of the LNT/SCR system may be stopped when a sensor response related to ammonia concentration peaks and then falls below a target value.
The inventor herein has recognized a potential inadequacy, however, in the approaches described above. Specifically, as the LNT catalyst ages, the amount of ammonia provided to the SCR device during the regeneration phase may be insufficient.
Therefore, the inventor has provided, in one embodiment, an emissions-control system of a vehicle that comprises an LNT disposed in an exhaust system of the vehicle, downstream of the engine, an SCR device disposed downstream of the LNT, and a controller. The controller is operatively coupled to the engine and configured to cause the engine to provide a reducing exhaust to the LNT at least during an enrichment interval, the enrichment interval including an enrichment subinterval, the controller further configured to adjust the enrichment subinterval in response to an ageing of the LNT. In one example, the enrichment interval may include a first enrichment subinterval commencing before rich breakthrough occurs in the LNT and ending before a second enrichment subinterval. The second enrichment subinterval commences after rich breakthrough occurs in the LNT and ends when the enrichment interval ends. Of course, these intervals may be varied and also additional enrichment subintervals may further be included, if desired.
In this way, it is possible to modify LNT regeneration based on catalyst performance and thereby generate sufficient ammonia for improved operation of the SCR device. Other embodiments provide additional emission-control systems and related methods to supply ammonia to an SCR device disposed downstream of an LNT in an exhaust system of a vehicle.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.